A technology for protecting a rider on a motorcycle by mounting an air bag apparatus on a motorcycle is known. For example, Japanese Unexamined Patent Application Publication No. 2001-219885 discloses a scooter-type motorcycle and a technology for inflating and expanding an air bag located in the space between a body component member such as a head pipe, etc. and a seat on which the rider sits. The technology suggests the possibility of providing with an air bag apparatus in the scooter-type motorcycle, but there also remains the requirement of a technological further study in the design of the air bag apparatus for a motorcycle while regarding the movement of the motorcycle in the case of the motorcycle-involved accident.